PIC18F6X2X datasheet(77/386 Pages) MICROCHIP

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 2003 Microchip Technology Inc.

Advance Information

DS39612A-page 75

PIC18F6X2X/8X2X

6.2

16-bit Mode

The

external

memory

interface

implemented

PIC18F8X2X devices operates only in 16-bit mode.

The mode selection is not software configurable but is

programmed via the configuration bits.

The WM1:WM0 bits in the MEMCON register deter-

mine three types of connections in 16-bit mode. They

are referred to as:

• 16-bit Byte Write

• 16-bit Word Write

• 16-bit Byte Select

These three different configurations allow the designer

maximum flexibility in using 8-bit and 16-bit memory

devices.

For all 16-bit modes, the Address Latch Enable (ALE)

pin indicates that the address bits A15:A0 are available

on the external memory interface bus. Following the

address latch, the Output Enable signal (OE) will

enable both bytes of program memory at once to form

a 16-bit instruction word. The Chip Enable signal (CE)

is active at any time that the microcontroller accesses

external memory, whether reading or writing; it is inac-

tive (asserted high) whenever the device is in SLEEP

mode.

In Byte Select mode, JEDEC standard FLASH memo-

ries will require BA0 for the byte address line and one

I/O line, to select between Byte and Word mode. The

other 16-bit modes do not need BA0. JEDEC standard

static RAM memories will use the UB or LB signals for

byte selection.

6.2.1

16-BIT BYTE WRITE MODE

Figure 6-1 shows an example of 16-bit Byte Write

mode for PIC18F8X2X devices. This mode is used for

two separate 8-bit memories connected for 16-bit oper-

ation. This generally includes basic EPROM and

FLASH devices. It allows table writes to byte-wide

external memories.

During a TBLWT instruction cycle, the TABLAT data is

presented on the upper and lower bytes of the

AD15:AD0 bus. The appropriate WRH or WRL control

line is strobed on the LSb of the TBLPTR.

FIGURE 6-1:

16-BIT BYTE WRITE MODE EXAMPLE

AD<7:0>

A<19:16>

ALE

D<15:8>

373

A<x:0>

D<7:0>

A<19:0>

A<x:0>

D<7:0>

373

WRH

WR(1)

Note

This signal only applies to table writes. See Section 5.1, “Table Reads and Writes”.

WRL

D<7:0>

(LSB)

(MSB)

PIC18F8X2X

D<7:0>

AD<15:8>

Address Bus

Data Bus

Control Lines

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